Current LCD screens with white LED backlight technology use a variety of configurations of the LED backlights. In these devices, the color of the picture is generated by opening and closing the LCD's; the brightness of the picture is provided from the LED backlight. The picture in front of the backlight can be switched off by disabling all of the backlight LED's; one section of the picture can be switched off by disabling the corresponding section of the backlight LED's; or all or part of the picture can be effectively dimmed by pulsing the corresponding backlight LED's on and off in a PWM fashion.
In many of these screens, the LED's are configured in multiple parallel strings, with each string containing more than one LED. In FIG. 1, for example, the illustrated configuration consists of 4 strings of ten LED's per string.
Each string also generally contains a current sink that controls the amount of current passing through each string. When the current through each string is equal to the current through every other string, each LED in the configuration produces the same light. That “same light” produced by one LED is consistent in terms of both brightness and color spectrum to the light produced by every other LED. FIG. 2 illustrates the same 4×10 configuration of LED's with the 4 associated current sinks. Together, we refer to one string of LED's and the associated current sink as one “channel.” FIG. 2 consists of four channels.
In most cases, the power supply of the appliance that contains the multiple channels does not produce the precise voltage that the channels require in order to operate, and the voltage must be stepped up or down by a DC-to-DC voltage converter. In order to maximize the efficiency of the voltage conversion, the converted voltage should be the smallest possible voltage that enables all of the LED strings to operate normally. For example, in the example of FIG. 2, if the 10 LED's have forward voltages that are centered around 3.5 volts, and the current sink has a voltage requirement of 1.0 volts, the supplied voltage that will enable each string to operate will be in the general neighborhood of 36 volts, derived as 10 times 3.5 volts plus 1.0 volts.
Unfortunately, the forward voltage requirements of the LED's are not uniform. Though the forward voltages may be centered about 3.5 volts (or some other voltage), the manufacturing process can lead to differences from one LED to the next. Hence, each channel can have different minimum operating voltage requirements, and even though many appliance manufacturers can adopt strategies to construct strings of LED's that are roughly similar in their forward voltage requirements, variations are inevitable.
Another issue occurs as the different channels are cycling on and off independently of one another. For example, if the channels are cycled on and off in a PWM fashion in order to dim all or part of the backlight, but different channels are cycled at different rates, the lowest forward voltage of the operating channels may vary from one instant to the next, and the converter may need to adjust its timing from one instant to the next. In addition, the changing current requirements of the channels cycling on and off can affect the performance of the aforementioned DC-to-DC converter, which can in turn affect the performance of the appliance's power supply. Both of these effects can affect the produced voltage, and should be fed back to the DC-to-DC converter in order to ensure efficient and accurate voltage supply.
In order to adjust the supplied voltage, the voltages of each operating (non-disabled) channel are monitored at the point between the LED's and the current sinks, as is illustrated in FIG. 3. As long as the voltage that is present at the measuring point is greater than the known voltage required by the current sinks, the channels will operate normally. It follows, then, that in order for all of the channels to operate normally, the voltage supplied to the strings by the converter must provide enough voltage at the point of measurement for each current sink to operate.